Coating formulations are commonly applied to a substrate by passing the coating formulation under pressure through an orifice into air in order to form a liquid spray, which impacts the substrate and forms a liquid coating. In the coatings industry, three types of orifice sprays are commonly used; namely, air spray, airless spray, and air-assisted airless spray.
Air spray uses compressed air to break up the liquid coating formulation into droplets and to propel the droplets to the substrate. The most common type of air nozzle mixes the coating formulation and high-velocity air outside of the nozzle to cause atomization. Auxiliary air streams are used to modify the shape of the spray. The coating formulation flows through the liquid orifice in the spray nozzle with relatively little pressure drop. Siphon or pressure feed, usually at pressures less than 18 psi, are used, depending upon the viscosity and quantity of coating formulation to be sprayed.
Airless spray uses a high pressure drop across the orifice to propel the coating formulation through the orifice at high velocity. Upon exiting the orifice, the high-velocity liquid breaks up into droplets and disperses into the air to form a liquid spray. Sufficient momentum remains after atomization to carry the droplets to the substrate. The spray tip is contoured to modify the shape of the liquid spray, which is usually a round or elliptical cone or a flat fan. Turbulence promoters are sometimes inserted into the spray nozzle to aid atomization. Spray pressures typically range from 700 to 5000 psi. The pressure required increases with fluid viscosity.
Air-assisted airless spray combines features of air spray and airless spray. It uses both compressed air and high pressure drop across the orifice to atomize the coating formulation and to shape the liquid spray, typically under milder conditions than each type of atomization is generated by itself. Generally the compressed air pressure and the air flow rate are lower than for air spray. Generally the liquid pressure drop is lower than for airless spray, but higher than for air spray. Liquid spray pressures typically range from 200 to 800 psi. The pressure required increases with fluid viscosity.
Air spray, airless spray, and air-assisted airless spray can also be used with the liquid coating formulation heated or with the air heated or with both heated. Heating reduces the viscosity of the liquid coating formulation and aids atomization.
A problem generally associated with airless spray techniques is their apparent inability to atomize the coating material in a manner so as to provide a desirable "feathered" pattern having a relatively wide central portion within which the distribution of coating material particles is reasonably uniform and opposite extremities or margins thereof having a progressively decreasing number of particles per unit of width. This is one of the principle reasons why, instead of using airless sprays, air sprays are used so as to provide coatings having high quality finishes. Air spray techniques, unlike airless spray techniques, are capable of producing a uniform amount of coating material particles at the center of the spray. This is particularly desirable so as to enable adjacent layers of the sprayed coating to be overlapped thereby providing a coating of uniform thickness.
The air spray technique is able to accomplish the production of a feathered spray pattern by using a large amount of compressed air to atomize the liquid coating formulation.
In contrast, airless spray techniques use no compressed air. Instead, the liquid coating formulation is sprayed at high pressure through an orifice into the environment in which the substrate to be coated is present, generally air. But airless spray guns characteristically give a fishtail spray pattern instead of a feathered spray pattern. Such fishtail spray patterns contain more coating material particles at the extremities or margins of the spray instead of in the center. Consequently, it is difficult to properly blend together adjacent layers of coating which have been deposited by means of such a fishtail spray pattern in a manner which provides a suitable uniform film of coating material upon a coated substrate.
However, it is well known that airless spray techniques deposit more of the coating formulation onto the substrate, i.e., it has a higher transfer efficiency, than that characterized by air spray techniques. As a result, while it is desirable to utilize airless spray techniques to obtain such higher transfer efficiencies, it is nevertheless hindered by the fact that it is known that the spray pattern that will be produced is one which does not provide a desirable feathered spray.
Accordingly, attempts have been made to obtain a desirable feathered spray pattern from airless spray techniques. Generally, such attempts have been focused upon mechanical solutions, namely, providing variations in the construction of the airless spray nozzles, providing turbulence promotion devices in the spray guns, combinations of these modifications, and the like. U.S. Pat. Nos. 3,659,787, 3,754,710, and 4,346,849 illustrate various embodiments in which the nozzle tip is constructed and modified so as to help provide a feathered spray. So too, in U.S. Pat. No. 3,556,411, a turbulence promoting device is described which is also included to help eliminate fishtailing in an airless spray technique.
While such mechanical improvements to the spray nozzle and/or spray gun may help improve the problem with fishtail patterns, they nevertheless suffer from the disadvantages of generally still not being able to provide a full, feathered spray pattern which is substantially equivalent to that obtained with air spraying. So too, they manifestly require the necessity of replacing and/or using specially constructed nozzle tips and/or spray guns in order to accomplish these objectives.
Clearly, what is needed is a means for providing a feathered spray pattern using an airless spray technique which feathered spray pattern is essentially equivalent of that obtained when using an air spraying technique. Moreover, such method should be capable of providing such a desirable feathered spray while using conventional airless spray guns and nozzles without the need to modify them in any manner.